Let
\[\mathbf{A} = \begin{pmatrix} 4 & 1 \\ -9 & -2 \end{pmatrix}.\]Compute $\mathbf{A}^{100}.$
Answer: Note that
\begin{align*}
\mathbf{A}^2 &= \begin{pmatrix} 4 & 1 \\ -9 & -2 \end{pmatrix} \begin{pmatrix} 4 & 1 \\ -9 & -2 \end{pmatrix} \\
&= \begin{pmatrix} 7 & 2 \\ -18 & -5 \end{pmatrix} \\
&= 2 \begin{pmatrix} 4 & 1 \\ -9 & -2 \end{pmatrix} - \begin{pmatrix} 1 & 0 \\ 0 & 1 \end{pmatrix} \\
&= 2 \mathbf{A} - \mathbf{I}.
\end{align*}Then $\mathbf{A}^2 - 2 \mathbf{A} + \mathbf{I} = 0,$ so
\[(\mathbf{A} - \mathbf{I})^2 = \mathbf{A}^2 - 2 \mathbf{A} + \mathbf{I} = \mathbf{0}.\]Thus, let
\[\mathbf{B} = \mathbf{A} - \mathbf{I} = \begin{pmatrix} 4 & 1 \\ -9 & -2 \end{pmatrix} - \begin{pmatrix} 1 & 0 \\ 0 & 1 \end{pmatrix} = \begin{pmatrix} 3 & 1 \\ -9 & -3 \end{pmatrix}.\]Then $\mathbf{B}^2 = \mathbf{0},$ and $\mathbf{A} = \mathbf{B} + \mathbf{I},$ so by the Binomial Theorem,
\begin{align*}
\mathbf{A}^{100} &= (\mathbf{B} + \mathbf{I})^{100} \\
&= \mathbf{B}^{100} + \binom{100}{1} \mathbf{B}^{99} + \binom{100}{2} \mathbf{B}^{98} + \dots + \binom{100}{98} \mathbf{B}^2 + \binom{100}{99} \mathbf{B} + \mathbf{I} \\
&= 100 \mathbf{B} + \mathbf{I} \\
&= 100 \begin{pmatrix} 3 & 1 \\ -9 & -3 \end{pmatrix} + \begin{pmatrix} 1 & 0 \\ 0 & 1 \end{pmatrix} \\
&= \boxed{\begin{pmatrix} 301 & 100 \\ -900 & -299 \end{pmatrix}}.
\end{align*}Note: We can expand $(\mathbf{B} + \mathbf{I})^{100}$ using the Binomial Theorem because the matrices $\mathbf{B}$ and $\mathbf{I}$ commute, i.e. $\mathbf{B} \mathbf{I} = \mathbf{I} \mathbf{B}.$  In general, expanding a power of $\mathbf{A} + \mathbf{B}$ is difficult.  For example,
\[(\mathbf{A} + \mathbf{B})^2 = \mathbf{A}^2 + \mathbf{A} \mathbf{B} + \mathbf{B} \mathbf{A} + \mathbf{B}^2,\]and without knowing more about $\mathbf{A}$ and $\mathbf{B},$ this cannot be simplified.